Copolymers of conjugated diolefin monomers and vinyl aromatic monomers tend to be blocky if they are prepared in a batch process without using a randomizing agent. Since a random distribution of repeat units which are derived from the conjugated diolefin monomer and the vinyl aromatic monomer is generally desirable, randomizing agents are very commonly utilized in the preparation of such polymers. Numerous randomizing agents which are commonly utilized in such polymerizations are disclosed in U.S. Pat. No. 4,311,803.
Many randomizing agents cause an increase in the vinyl content of the polymer being prepared. For this reason vinyl content and blockiness cannot be controlled independently with such randomizers. In situations where it is desirable to prepare a random copolymer without increasing vinyl content it is necessary to select a randomizer which does not increase vinylation or which only increases vinylation to a minimal extent. Tertiary-butyl potassium is a randomizing agent which causes increased vinylation which may be used commercially. However, the use of tertiary-butyl potassium presents a fire hazard due to its pyrophoric nature and it is not very soluble in the organic solvents commonly utilized as the medium for conducting such polymerizations.